1. Field of the Invention
This invention pertains in general to the nondestructive examination of high pressure turbine outer cylinders and more particularly to the nondestructive examination of the main steam inlet sleeves on Westinghouse high pressure outer cylinders and nozzle chamber to cylinder welds on the high pressure inner cylinders, employing linear phased ultrasonic transducer arrays.
2. Related Art
Westinghouse and its subsidiaries have been inspecting the steam inlet components of Westinghouse turbine high pressure cylinders since the 1970s employing as its primary means of inspection of the trepan region of the main steam inlet nozzle a magnetic rubber inspection process. The main areas of concern are the trepan region of the main steam inlet sleeves on the high pressure outer cylinder and the nozzle chamber to cylinder welds on the high pressure inner cylinder. These regions are the areas of maximum stress build-up. The nozzle to inner cylinder welds are inspected using flexible fiber optics, ultrasonic transducers and a radiological examination process.
The magnetic rubber inspection process requires two days of examinations plus a day for dust blasting. The process also requires two 440 volt set-ups for the magnetizing equipment employed to induce a magnetic field on the area to be inspected. The magnetic rubber inspection process provides a good surface examination but the sensitivity is dependent on the adequacy of the magnetic field that is induced that cannot be accurately verified.
The magnetic rubber inspection technique uses a liquid (uncured) rubber containing suspended magnetic particles. The rubber compound is applied to the area to be inspected on a magnetized component. Inspections can be performed using either an applied magnetic field, which is maintained while the rubber sets, or a residual field from magnetization of the component prior to or shortly after pouring the compound. This latter technique is employed in inspecting the Westinghouse turbines. A dam of modeling clay, or an inflatable dam such as that described in U.S. Pat. No. 4,598,580, is often used to contain the compound in the region of interest. The rubber is allowed to set, which takes approximately two hours, and then the rubber cast is removed from the part. The rubber conforms to the surface contours and provides a reverse replica of the surface. The rubber cast is examined for evidence of discontinuities, which appear as lines on the surface of the molding. The molding can be retained as a permanent record of the inspection.
Magnetic particle inspection uses the tendency of magnetic lines of force, or flux, of an applied field to pass through the metal rather than through the air. A defect at or near the metal surface distorts the distribution of the magnetic flux and some of the flux is forced to pass out through the surface. The field strength is increased in the area of the defect and opposite magnetic poles form on either side of the defect. Fine magnetic particles within the uncured rubber solution applied to the part are attracted to these regions and form a pattern around the defect. The pattern of particles is maintained by the induced magnetic field until the rubber cures and permanently captures the pattern as a visual indication of a defect. Operators employing magnetic particle inspection have to recognize nonrelevant error indications during examination. Proper analysis of indications in these regions will require considerable skill and experience and supplemental examination methods are required before a final evaluation can be made. The technique cannot be used to determine flaw depth which further complicates interpretation of the indications.
A visual inspection of the nozzle chamber to inner cylinder welds is made using flexible fiber optics to examine the root of the welds for service induced cracks; ultrasonic transducers are used to further verify cracks on the sheer plane emanating from the weld fillet radius of the trepan; and a radiological examination is performed to identify cracks running radially into the weld or adjacent areas.
The time to perform these inspections typically takes two to three days and can fall within the critical path of an outage. The radiological portion of the examination requires that the cylinder has to be isolated from all other work being performed and thus can delay work being performed in parallel. The work can be further held up if the equipment to perform the radiological exam or to magnetize the part is not timely available. Furthermore, these examinations do not provide crack depth information which complicates the characterizations of aberrations on the surface that might be noted, as flaws induced by fatigue.
Any fatigue-induced crack formed in either the trepan region of the sleeves or the nozzle chamber to cylinder welds necessitates replacement of the main steam inlet nozzle which can cost in the order of magnitude of $250,000 and significantly extend an outage. Therefore, it is important that aberrations detected during the nondestructive examination be properly characterized.
Accordingly, it is an object of this invention to provide an improved nondestructive examination process for identifying cracks in the main steam inlet nozzle sleeves and nozzle chamber to cylinder welds of high pressure turbines that does not require surface preparation such as the dust blasting step required for the magnetic rubber inspection process.
It is a further object of this invention to provide an improved inspection technique that significantly reduces the time to inspect the main steam inlet nozzle sleeves and nozzle chamber to cylinder welds of high pressure turbines.
It is a further object of this invention to provide such an improved nondestructive examination procedure that will identify the depth of any anomaly noted in the main steam inlet nozzle sleeves and the nozzle chamber to cylinder welds of high pressure turbines that will permit more accurate characterization of the anomalies as fatigue-induced flaws.